But it gets better than that. If your partner is also your best
friend, you don't actually need to be married to reap the
benefits of the relationship.

The increased happiness levels the researchers found to
be linked with marriage held true for best-friend couples who
lived together too, even if they weren't married.

To arrive at their conclusions, the researchers studied three
separate data sets that included information about thousands of
couples: The United Kingdom's Annual Population Survey, the
British Household Panel Survey, and the Gallup World Poll. Then,
they controlled for couples' age, gender, income, and health
conditions (all of which could potentially affect their results).

Here's a chart from the paper comparing the life satisfaction of
people who had ever been married (red line) with people who were
single and had never been married (blue line):

There's something crucial missing from that chart though: The
results were very similar for cohabitating couples who considered
their partner their best friend but were not married.

Here's a chart comparing the life satisfaction of couples who
were married (blue bars) with couples who lived together but were
unmarried (red bars). Couples whose partner was also their best
friend are to the left; couples who had another best friend who
was not their partner are to the right.

People in a relationship who saw their significant other
as their best friend and either lived with that person or married
them were happier than couples who saw their best friend as
someone outside of the relationship.

"What immediately intrigued me about the results was to rethink
marriage as a whole," University of British Columbia economics
professor and study coauthor
John Helliwell told the New York Times. "Maybe what
is really important is friendship, and to never forget
that in the push and pull of daily life."

This takeaway squares with other research. A
2012 survey of American couples found that those who lived
together but were not married had higher self-esteem and were
happier overall than their married counterparts, even though both
types of relationships improved bigger-picture well-being. Other
studies have shown too that, despite persistent narratives about
marriage as key to happiness, tying the knot doesn't always have
a net positive effect on couples. A 2011 review of the
impact on happiness of major life events found that couples
who got married generally felt less happy and less satisfied with
their lives over time.

In other words, your significant other should be your best
friend. But as far as marrying that person goes? Not required for
optimal happiness.